This invention relates to preparation of metal halides. More particularly, this invention relates to a process for preparing high purity metal fluorides.
Work on the purification of metal fluorides has intensified greatly in recent years because of interest in such applications of heavy metal fluoride glasses as optical fibers, tunable solid state lasers, and dielectric layers for complex semiconductor structures. Initially, researchers made rapid advances in increasing the purity of these materials. Recently, however, progress has slowed although impurity levels remain orders of magnitude higher than can be tolerated in some applications.
For example, heavy metal fluoride glasses theoretically have the potential for providing extraordinarily low loss optical fibers operating in the 2-4 .mu.m region of the infrared. One of the most serious hindrances to achieving such low loss fibers is contamination of the glass components with certain divalent transition metals, some rare earth ions, and the hydroxyl ion. These impurities have strong absorptions in the optimum range for optical applications. To exploit the potential for these glasses, it is necessary to bring such impurity levels to as low as one part per billion for some of these contaminants.
A number of purification approaches are presently used, including wet chemical processing and vapor treatment. The well known wet processing method has limitations in its degree of purification because of recontamination from the background levels of contaminants present in the processing chemicals. Most current vapor preparation techniques are limited to dealing with the existing fluoride compound, and suffer from low or non-existent thermodynamic driving force for removal of the contamination. The "reactive atmosphere process" (RAP), used to purify a number of starting materials, has an important effect on the hydroxyl content, but rather little effect on the other contaminants. Physical vapor transport (sublimation) has been used to purify ZrF.sub.4. AlF.sub.3, BaF.sub.2, and GdF.sub.3 have been purified by subliming the transition metals out of them, showing greater than an order of magnitude improvement in Fe.sup.2+, but little effect on the other contaminant ions. Sublimation is limited by the ratio of the vapor pressure of the contaminant species to that of the desired compound.
One of the basic limitations of the above processes is that the starting materials are often of limited purity. Thus the process is required to provide more purification than is reasonable. Additionally, there are fundamental practical limitations on the amount of the contaminants that can be removed because of similarities in vapor pressures of the contaminant compound and its solid solution with the major compound.
Above-referenced U.S. Pat. No. Re. 32,777 describes a chemical vapor purification (CVP) process in which the desired metal ion is selectively extracted because of thermodynamic partitioning between the desired metal cation and the contaminating metal and hydroxyl cations. A reactive transport agent, such as chlorine, bromine, or iodine, is reacted with the desired metal and generates a gaseous metal-containing compound. The gaseous metal-containing compound, containing the highly purified metal cation, is then isolated from the starting materials, which include the excess unreacted metal and any contaminants. The gaseous metal-containing compound is then reacted with a fluorinating agent to form the desired solid metal fluoride in a step that further enhances the purification of the metal fluoride compound. This process, for most metal fluorides for which it is intended, is a great improvement over prior art processes.
However, the presence of a very stable barrier layer of "native" oxides on the surface of certain thermodynamically partitionable metals, for example aluminum, can interfere with the reactions that are fundamental to the above-described CVP process. For the most efficient operation of the process, the reactive transport agent used to form the purified vapor must come into intimate contact with non-oxidized metal over a reasonably large surface area. Also for efficient operation, the volatile compounds formed in the initial reaction between the metal and the reactive transport agent should not be impeded by a surface layer of oxidized metal from being transported from the metal surface.
A typical CVP process for purifying metal involves reacting chlorine with aluminum metal at a temperature above the sublimation temperature of AlCl.sub.3 and its dimer Al.sub.2 Cl.sub.6, i.e. above 183.degree. C. at atmospheric pressure, as shown in Reaction 1: EQU 2Al+3Cl.sub.2 .rarw..fwdarw.Al.sub.2 Cl.sub.6 .uparw. (1)
The reaction is carried out under process conditions which thermodynamically and kinetically sufficiently strongly favor the formation of the desired halide to cause Reaction (1) or its equivalent to to proceed essentially to completion, and thus nearly to equilibrium.
Normally the chlorine reacts vigorously with the aluminum metal where they first come into contact. As the reaction proceeds, more Al.sub.2 Cl.sub.6 is formed and the active chlorine gas at the metal surface becomes more dilute The reaction thus can become less vigorous.
If, as can occur in the process described in above-referenced U.S. Pat. No. Re. 32,777, a stable oxide barrier layer were formed, residual chlorine could be left unreacted in the vapor stream mixed with the Al.sub.2 Cl.sub.6. In the case of a particularly stable surface barrier, little Al.sub.2 Cl.sub.6 may be formed. Thus, when a solid source metal is used, maintaining sufficient exposed surface area for efficient operation may be difficult.
The chlorine, on contact with the aluminum, also attacks the impurities in the aluminum, forming chlorides of these impurities by reactions similar to Reaction 1. To ensure complete partitioning of the impurities from the desired metal, Reaction 1 must be followed by a second reaction to reduce these volatile chlorides of the impurity elements. An example of this reduction is illustrated by Reaction 2, using nickel as the impurity: EQU NiCl.sub.2 +2/3Al.rarw..fwdarw.Ni.dwnarw.+1/3Al.sub.2 Cl.sub.6 .uparw.(2)
Similar reactions are required for the partitioning of other undesired compounds of the 3d transition metal elements such as Fe, Co, and Cu. This second reaction is the actual purification step, since the impurity is reconverted to a low vapor pressure metal form leaving a purified stream of the desired metal chloride. The presence of sufficient exposed metal surface to stimulate the reduction of the volatile halide of the impurity to its metallic form, as shown in Reaction 2, is thus also important to an efficient CVP process.
Reaction 2 has a smaller favorable free energy difference than Reaction 1, thus is not as favored as Reaction 1 if there is an interfering layer on the surface of the metal. The required exposure of the base metal to the reactants can be diminished or even eliminated when the metal surface is coated with an oxide barrier layer.
The barrier layer impeding the reactions is a native oxide film that forms almost instantly when some metals, for example aluminum, are exposed to ambient air. The oxides are formed by the reaction of atmospheric water vapor and oxygen with the base metal. This well known reaction begins within milliseconds after a fresh surface of aluminum metal is exposed to air. The oxides continue to form until a stable oxide barrier layer is present, having sufficient thickness to prevent further exposure of the metal to the ambient atmosphere. In the case of aluminum, the film is very stable and tenacious, and is nearly free of flaws. Since Reaction 3, below, favors the formation of Al.sub.2 O.sub.3 over that of Al.sub.2 Cl.sub.6, it is only at such flaws that chlorine gas can penetrate the oxide barrier to react with the underlying metal: EQU Al.sub.2 O.sub.3 +3Cl.sub.2 .rarw..fwdarw.Al.sub.2 Cl.sub.6 +3/2O.sub.2( 3) ps
The equilibrium mole fractions of components in the system Al.sub.2 O.sub.3 /Cl.sub.2 are listed in Table I. These equilibrium mole fractions show that the reverse of Reaction 3, the formation of the oxide, is highly favored thermodynamically over the range of temperatures which is most desirable for CVP processing. (Table I is based on input conditions of one mole of Al.sub.2 O.sub.3 and two moles of Cl.sub.2. Results are shown in Table I only if the mole fraction was at least 10.sup.-10 for at least one temperature.)
TABLE I ______________________________________ Equilibrium Mole Fractions @ Temperature, kelvin Phase 873 773 673 573 473 ______________________________________ Al.sub.2 Cl.sub.6 1.1.sup.-08 6.9.sup.-10 0.0 0.0 0.0 Al.sub.2 O.sub.3 (s) 3.3.sup.-01 3.3.sup.-01 3.3.sup.-01 3.3.sup.-01 3.3.sup.-01 Cl 2.9.sup.-05 3.2.sup.-06 1.9.sup.-07 4.1.sup.-09 1.8.sup.-11 ClO 4.2.sup.-10 1.8.sup.-11 0.0 0.0 0.0 Cl.sub.2 6.7.sup.-01 6.7.sup.-01 6.7.sup.-01 6.7.sup.-01 6.7.sup.-01 O.sub.2 1.6.sup.-08 1.0.sup.-09 0.0 0.0 0.0 ______________________________________
Thus when an oxide surface layer is present removal or penetration of the layer by reaction with chlorine alone is not favored, and the equilibrium level of the dimeric form of aluminum chloride is very minute. The ability of this barrier layer to readily reestablish itself when re-exposed to air adds to the difficulty of optimizing the CVP process for metals tending to form such layers.
Removal of the native oxide by chemical or mechanical means requires an extra step, and can be ineffective because the oxide is reestablished so rapidly.
The present invention provides an improved CVP process for purifying metals, which increases the efficiency of the process by permitting unimpeded contact between the reactive transport agent and the metal and by preventing trapping of the resulting gaseous compounds by surface barrier layers.